Process for synthesizing a zeolite catalyst on a pH controlled basis to improve catalyst life

Abstract

A process for preparing a zeolite, e.g. ZSM-5, under controlled conditions of initial pH adjustment with an acid, SiO2/Al2O3 mole ratio, and in the presence of sodium cations, is disclosed, as well as a process for using the same to synthesize olefins from methanol and/or dimethyl ether.

What is claimed is: 
     
       1. A process for producing a crystalline alumino-silicate zeolite which comprises: (1) admixing to form a reaction mixture in the absence of a buffer: (a) at least one tetraalkyl ammonium hydroxide in an amount sufficient to impart a pH to the reaction mixture in the initial absence of said acid of (f) of not less than 13 when measured at room temperature;   (b) at lest one sodium cation source;   (c) at least one silica source;   (d) at lest one alumina source;   (e) water; and   (f) at least one organic acid in a manner and under such conditions that the reaction mixture prior to acid addition has a composition within the following molar ratio ranges:   ______________________________________                                    
(TAA).sub.2 O/SiO.sub.2                                                   
               0.01 to       5                                            
SiO.sub.2 /Al.sub.2 O.sub.3                                               
               80 to         2200                                         
H.sub.2 O/SiO.sub.2                                                       
               10 to         200                                          
Na.sub.2 O/SiO.sub.2                                                      
               0.05 to       5                                            
______________________________________                                    
     wherein TAA is the sum of the tetraalkyl ammonium cations; the initial pH of the reaction mixture, when measured at room temperature, being adjusted with said acid to be from about 11.3 to about 11.7;       (2) heating the reaction mixture until crystals of said zeolite form; and   (3) separating said crystals from the reaction mixture.   
     
     
       2. The process of claim 1 wherein said tetraalkylammonium hydroxide is tetrapropylammonium hydroxide. 
     
     
       3. The process of claim 2 wherein the silica source and sodium cation source is sodium silicate. 
     
     
       4. The process of claim 3 wherein the alumina source is aluminum nitrate. 
     
     
       5. The process of claim 2 wherein the initial pH of the reaction mixture is adjusted with acid to between about 11.4 and about 11.6. 
     
     
       6. The process of claim 2 wherein the acid is HNO 3 . 
     
     
       7. The process of claim 1 wherein the pH of the initial reaction mixture in the absence of said acid is 14, and sufficient acid is added to reduce the initial pH of the reaction mixture to 11.5. 
     
     
       8. The process of claim 1 wherein the SiO 2  /Al 2  O 3  mole ratio in the resulting zeolite is from about 100 to about 1000. 
     
     
       9. The process of claim 1 wherein the separated zeolite crystals are converted to the hydrogen form by cation exchange and calcination. 
     
     
       10. The process of claim 1 wherein said reaction mixture is heated to a temperature of from about 100° to about 220° C. for a period of from about 3 to 8 days. 
     
     
       11. A process for producing a crystalline aluminosilicate ZSM-5 zeolite which comprises: (A) providing in the absence of a buffer, a reaction mixture comprising tetrapropylammonium hydroxide, a silica source, an alumina source, a sodium cation source, and water, said reaction mixture components being present in amounts sufficient to impart: (i) an initial pH to the reaction mixture of not less than about 13; and (ii) a composition to the reaction mixture expressed in terms of mole ratios comprising:   ______________________________________                                    
(TPA).sub.2 O/SiO.sub.2                                                   
               0.05 to       1                                            
SiO.sub.2 /Al.sub.2 O.sub.3                                               
               220 to        800                                          
H.sub.2 O/SiO.sub.2                                                       
               20 to         140                                          
OH.sup.- /SiO.sub.2                                                       
               0.2 to        5                                            
H.sub.2 O/OH.sup.-                                                        
               10 to         200                                          
Na.sub.2 O/SiO.sub.2                                                      
               0.1 to        4                                            
______________________________________                                    
     wherein TPA signifies tetrapropylammonium;     (B) adjusting the initial pH of the reaction mixture to between about 11.3 and about 11.7 when measured at room temperature, by the addition of nitric acid;   (C) heating the pH adjusted reaction mixture in a manner and under conditions sufficient to cause crystals of said zeolite to form; and   (D) separating said crystals from the reaction mixture.   
     
     
       12. The process of claim 11 wherein said crystals are exchanged with an ammonium cation to replace sodium cations present therein and calcined at a temperature of from about 400° to about 900° C. in a manner sufficient to convert the zeolite to the hydrogen form. 
     
     
       13. The process of claim 11 wherein in said reaction mixture the Na 2  O/SiO 2  mole ratio is from about 0.3 to about 3. 
     
     
       14. The process of claim 11 wherein the alumina source is aluminum nitrate, and the silica and sodium cation source is sodium silicate. 
     
     
       15. The process of claim 11 wherein the composition of the reaction mixture is sufficient to impart a SiO 2  /Al 2  O 3  mole ratio to the crystalline zeolite of from about 200 to about 650. 
     
     
       16. The process of claim 11 wherein the pH adjusted reaction mixture is heated to a temperature of from about 120° to about 190° C. for a period of from about 3 to about 8 days. 
     
     
       17. The process of claim 16 wherein the reaction mixture is agitated for at least 24 hours during said heating. 
     
     
       18. The process of claim 16 wherein said heating is conducted under an inert atmosphere at a pressure of from about 5 to about 100 psi.